But government officials and nuclear experts have also expressed fears to the Financial Times about what may be a significant stockpile of uranium inside Syria.

Concerns go back to the Assad regime’s attempt to build a nuclear reactor at Al-Kibar in the east of the country. Syria, with assistance from North Korea, was thought to be close to completing the reactor when the facility was destroyed by Israeli jets in September 2007.

Very little is known about Syria’s nuclear programme and the country has always denied that it ever had one. Syria has also given very little information to the International Atomic Energy Agency, the nuclear watchdog, about its work.

However, intelligence officials and experts have long believed that the reactor was of a similar design to the one that North Korea has built at Yongbyon. Experts have concluded, from a comparison of the two reactors, that Al-Kibar would have needed about 50 tonnes of natural uranium fuel to become operational.

“There are real worries about what has happened to the uranium that Syria was planning to put into the Al-Kibar reactor shortly before the reactor was destroyed in 2007,” he said. “There’s no question that, as Syria gets engulfed in civil war, the whereabouts of this uranium is worrying governments. There is evidence to suggest this issue has been raised by one government directly with the IAEA.”

An IAEA inspection team visited the destroyed Al-Kibar site in May 2008 and only found traces of uranium. This merely added to the mystery of where the 50 tonnes of uranium, if it exists, might be. Such a stockpile would be enough, according to experts, to provide weapons grade fuel for five atomic devices.

Some government officials have raised concerns that Iran, which is closely allied to the Syrian regime and urgently needs uranium for its nuclear programme, might be trying to seize such a stockpile.

These officials’ fears have been triggered by signs of movement at what they allege is a secret uranium conversion facility that the Syrian regime built at the town of Marj al-Sultan near Damascus.

Three satellite pictures of the Marj al-Sultan site taken in October, November and December of 2012 and shown to the FT, and displayed here above and on the left, appear to show the gradual clearance of a large orchard there, for no apparent reason.

Whether the uranium is at the site is unclear, the officials conceded. But they said: “Syria is almost certainly in possession of good quality uranium of the type that Iran has been trying to acquire on the international market for years. It would certainly be possible to transfer this from Syria to Iran by air.”

Were that to happen – and Iran were to attempt to build another secret uranium plant – such a stockpile could be a “vital resource”, the officials argued, and possibly be used to build a bomb.

Other officials raised doubts over such a scenario involving Iran. Syria may have a stockpile of 50 tonnes of pure uranium, one European official said. But whether it is at serious risk of seizure by Iran is another matter, the official added. The IAEA declined to comment.